This invention relates to antennas, phased array antennas, and specifically to a stacked dual-band electromagnetic band gap (EBG) waveguide aperture with independent feeds.
Electronically scanned arrays or phased array antennas offer significant system level performance enhancements for advanced communications, data link, radar, and SATCOM systems. The ability to rapidly scan the radiation pattern of the ESA allows the realization of multi-mode operation, LPI/LPD (low probability of intercept and detection), and A/J (antijam) capabilities. One of the major challenges in ESA design is to provide cost effective antenna array phase shifting methods and techniques along with dual-band operation of the ESA.
It is well known within the art that the operation of a phased array is approximated to the first order as the product of the array factor and the radiation element pattern as shown in Equation 1 for a linear array.
                                          E            A                    ⁡                      (            θ            )                          ≡                                                            E                p                            ⁡                              (                                  θ                  ,                  ϕ                                )                                                    ︸                              Radiation                                  Element                  Pattern                                                              ⁢                                                                      [                                                            exp                      ⁡                                              (                                                  -                                                      j                                                                                          2                                ⁢                                                                  π                                  o                                                                                            λ                                                                                                      )                                                                                    r                      o                                                        ]                                ︸                                            Isotropic                                  Element                  Pattern                                                      ·                                                            ∑                  N                                                                                        ⁢                                                                  ⁢                                                      A                    n                                    ⁢                                      exp                    ⁡                                          [                                                                        -                          j                                                ⁢                                                                              2                            ⁢                                                                                                                  ⁢                            π                                                    λ                                                ⁢                        n                        ⁢                                                                                                  ⁢                        Δ                        ⁢                                                                                                  ⁢                                                  x                          ⁡                                                      (                                                                                          sin                                ⁢                                                                                                                                  ⁢                                θ                                                            -                                                              sin                                ⁢                                                                                                                                  ⁢                                                                  θ                                  o                                                                                                                      )                                                                                              ]                                                                                                  ︸                                  Array                  ⁢                                                                          ⁢                  Factor                                                                                        Equation        ⁢                                  ⁢        1                            θ=angle of beam scanning (steering) to the far field observation point as referenced to the nominal beam angle, as described by the array coordinate system. This is typically the angle from an axis normal (perpendicular) to the array face. It is often referenced from the z axis of a right-handed spherical coordinate system and often describes the “elevation angle” of the array main beam relative to its nominal position.        φ=the angle referenced from the x axis of a right handed spherical coordinate system and often describes the “azimuth angle” of the array main beam relative to its nominal position.        j=√(−1) the imaginary number operator        λ=the free space wavelength of the signal radiated by the linear array        π=the mathematical constant 3.14159 . . .        ro=the radial distance from the array center to the far field observation point        An=the relative amplitude weighting of each element within the linear array        n=the number of radiating elements in the linear array        Δx=the physical spacing between each element in the linear array        θ0=the angle of the array's nominal beam position, as point referenced to the array coordinate system. It is usually the angle referenced of the z axis of a right-handed spherical coordinate system. This is the reference angle in which the amount of beam scanning, as described by θ, is referenced, and is typically 0° or 90° in application.        
Standard spherical coordinates are used in Equation 1 and θ is the scan angle referenced to bore sight of the array. Introducing phase shift at all radiating elements within the array changes the argument of the array factor exponential term in Equation 1, which in turns steers the main beam from its nominal position. Phase shifters are RF devices or circuits that provide the required variation in electrical phase. Array element spacing is related to the operating wavelength and sets the scan performance of the array. All radiating element patterns are assumed to be identical for the ideal case where mutual coupling between elements does not exist. The array factor describes the performance of an array of isotropic radiators arranged in a prescribed two-dimensional rectangular grid.
A packaging, interconnect, and construction approach is disclosed in U.S. Pat. No. 6,822,617 that creates a cost-effective EMXT (electromagnetic crystal)-based phased array antenna having multiple active radiating elements in an X-by-Y configuration. EMXT devices are also known in the art as tunable photonic band gap (PBG) and tunable electromagnetic band gap (EBG) substrates. A description of a waveguide section with tunable EBG phase shifter technologies is available in a paper by J. A. Higgins et al. “Characteristics of Ka Band Waveguide using Electromagnetic Crystal Sidewalls” 2002 IEEE MTT-S International Microwave Symposium, Seattle, Wash., June 2002 and U.S. Pat. No. 6,756,866 “Phase Shifting Waveguide with Alterable Impedance Walls and Module Utilizing the Waveguides for Beam Phase Shifting and Steering” by John A. Higgins. Each element is comprised of EMXT sidewalls and a conductive (metallic) floor and ceiling. Each EMXT device requires a bias voltage plus a ground connection in order to control the phase shift for each element of the antenna by modulating the sidewall impedance of the waveguide. By controlling phase shift performance of the elements, the beam of the antenna can be formed and steered.
Phase shifter operation in dual modes in one common waveguide with independent phase control for each mode at the same or different frequency bands for phased array antennas and other phase shifting applications is a desirable feature to increase performance and reduce cost and size. Dual bands of current interest include K Band (20 GHz downlink) and Q Band (44 GHz uplink) for satellite communication (SATCOM) initiatives. The EBG ESA must be able to perform at two significantly different frequencies.
Dual-band EBG ESA antennas are constructed of square EBG waveguide phase shifters. The waveguide aperture size is determined so as to maximize phase shift while minimizing loss. Smaller apertures yield greater phase shift per unit length, but higher loss due to input mismatch. As the frequencies of a dual-band EBG ESA are made further apart, the task of achieving low-loss 360° phase shifter performance becomes daunting. Dual-band EBG 360° analog waveguide phase shifters for use in ESA antenna apertures are difficult to design due to the difference in performance tradeoffs encountered at each frequency.
What is needed is a low-cost, low-loss, dual-band EBG ESA waveguide antenna utilizing techniques that enable dual frequency operation, especially in the case of significantly different operating frequencies.